This invention relates to a method of evaluating a tunnel insulating film, and in particular, to a method of evaluating a tunnel insulating film of an MOS FET (metal oxide semiconductor field-effect transistor) having a dual gate structure, for example a cell transistor of a flash memory device.
FIGS. 1a and 1b are sectional views showing a conventional dual gate MOS FET used as a cell transistor of a flash memory device. FIG. 1a is a vertical section along the direction of source and drain regions. FIG. 1b is a vertical section along the direction perpendicular to FIG. 1a.
In the MOS FET shown in FIGS. 1a and 1b, a tunnel insulating film 105 and a field insulating layer 111 are formed on a monocrystalline silicon substrate 101. A floating gate 106, a silicon insulating layer 107, a control gate 108 and an insulating layer 109 are deposited on the tunnel insulating film 105 and the field insulating layer 111 one after another. A metal layer 110 is formed on the insulating layer 109 and connected to the control gate 108. A channel region 104 is formed in the surface of the substrate 101 between a source region 103 and a drain region 102 under an applied voltage. W1 in FIG. 1b shows the width of the channel region 104.
Data can be erased by injecting electrons from the channel region 104, the source region 103 and the drain region 102 into the floating gate 106 through the tunnel film 105. Data can he written by discharging the electrons from the floating gate 106 into the drain region 102 through the tunnel film 105. The flow of the electrons through the tunnel film 105 is called "Fowler/Nordheim (FN) tunnel current". The threshold voltage of the MOS FET in an erasing state (Vte) is higher than the threshold voltage in a writing state (Vtw). The difference between Vte and Vtw is called the Vt Window.
In such an MOS FET, the Vte falls and the Vtw rises, namely, the Vt Window decreases as the result of rewriting data over and over again. The decrease of the Vt Window prevents the data from judging exactly. It is thought that the threshold voltages change because electrons and holes are trapped within the tunnel film or the interface state formed between the tunnel film and the semiconductor substrate. Since the degree of changes of the threshold voltages depend on the quality of the tunnel film (the better the quality of the tunnel film is, the smaller the changes in the threshold voltages after rewriting data over and over again), it is important to evaluate the quality of the tunnel film in order to establish the electrical characteristics and reliability of the flash memory.
The quality of the tunnel film is conventionally evaluated by measuring the changes in the threshold voltages (Vte and Vtw) after rewriting data over and over again.
FIG. 2 is a flowchart which shows conventional steps carried out to evaluate the quality of the tunnel film. In this figure, first, Vtw is measured before rewriting data (step 121). Next, the data is erased (step 122) and Vte is measured (step 123). As an example, data is erased by applying a control gate voltage of 18V and a source voltage of 0V to the device with respect to the silicon substrate 101 having a voltage of 0V, and disconnecting the drain. Next, data is written (step 124) and Vtw is measured (step 125). As an example, data is written by applying a drain voltage of 5V and a control voltage of -8V to the device with respect to the silicon substrate 101 having a voltage of 0V and disconnecting the source. Then steps 122 to 126 are repeated.
Steps 122 to 126 are repeated a predetermined number of times, and then the quality of the tunnel film is evaluated from the relation between the number of times rewriting occurred and the Vt Window. Compared with the number of rewriting times and the smaller the decrease in the Vt Window, the better the quality of the tunnel film.
In this case it takes a relatively long time to evaluate the quality of the tunnel film because the data is actually rewritten. For example, it takes 1 hour to repeat rewriting one hundred thousand times. Further, sometimes the Vt Window fails to change in spite of being rewritten one hundred thousand times.